1. Field of the Invention
The present invention relates to wavelength actuators used in laser cavities. Certain embodiments relate to holographic gratings and liquid crystal switchable gratings.
2. Description of the Related Art
Tunable lasers are very useful in the optical networking and telecommunications industry. Tunable lasers provide the benefit over fixed-wavelength lasers of allowing one laser to produce light at a variety of wavelengths. A tunable laser allows selection or tuning of the laser to a specific wavelength or a specific range of wavelengths. Some examples of tunable lasers include, but are not limited to, distributed-feedback lasers (DFBs), distributed-Bragg reflector lasers (DBRs), vertical-cavity semiconductor lasers (VCSELs) that employ microelectromechanical systems technology, and external-cavity diode lasers (ECLs).
In an ECL, at least some portion (e.g., wavelength tuning components) of the laser cavity resides off the laser-emitting device (e.g., a laser diode). ECLs may provide a wide range for wavelength tuning (e.g., about 100 nm or greater, or from the visible range to the infrared range), very narrow line widths, and very high output powers relative to other tunable lasers.
Several different methodologies may be used for wavelength selection and wavelength tuning in ECLs. Wavelength selection and tuning methodologies include, but are not limited to, mechanical tuning, thermal tuning, and/or electro-optical tuning. Mechanical tuning involves the use of motors and/or actuators to mechanically move parts in a laser cavity. Thermal tuning involves the movement of parts by direct thermal expansion or direct thermal contraction of materials in the laser cavity. Mechanical and thermal tuning may have problems associated with repeatability of the tuning and/or stability in the tuning due to the movement of parts. In addition, minimum tolerances may be difficult to maintain in mechanical and thermal tuning due to moving parts. Electro-optical tuning employs a crystal or a solid-state device that allows for electrical manipulation of the optical properties of the crystal or solid-state device.
Wavelength selection and tuning are important for selectable single frequency operation in a laser cavity. Selectable single frequency operation may be accomplished with a narrowband filter in a laser cavity. For example, a diffraction grating may be used for band filtering in a laser cavity.
FIG. 1 depicts a schematic of an embodiment of an external cavity laser configuration. Laser emitting device 102 may be coupled to lens 104 and diffraction grating 106. Lens 104 may be a collimating lens. The laser cavity may produce laser light output 108. Diffraction grating 106 along with retroreflector 110 may be used to select and/or tune a wavelength of laser light output 108.
Filtering in the laser cavity may be used to select an axial mode of the laser light. Cavity length of the laser cavity may also be altered to select an exact laser light wavelength. FIG. 2 depicts schematics of embodiments for wavelength tuning including grating angle tuning (FIG. 2A) and cavity length tuning (FIG. 2B). In FIGS. 2A and 2B, the dashed vertical lines depict the axial modes of the cavity while the solid line depicts the cavity filter function. In each tuning method, the cavity filter initially selects an axial mode (e.g., axial mode q, as shown in FIG. 2).
Filtering typically involves changing the angle of a diffraction grating to shift a center wavelength of the cavity filter, as shown by the arrow in FIG. 2A. Cavity length tuning involves changing the optical path length in the cavity. Mechanically moving parts to adjust the path length or altering an index of refraction of a material along the optical path may change the optical path length. Changing the optical path length moves the axial modes (e.g., the dashed lines in FIG. 2B), which allows different axial modes to be selected.
Some tunable lasers (e.g., external cavity lasers) may be susceptible to mode hopping. Mode hopping may result from a mismatch between a change in the resonant wavelength of the laser cavity and an accompanying change in the optical path length of the laser cavity. For example, such a mismatch may upset a mathematical ratio that controls the design of the laser cavity and cause the laser to hop one mode. Mode hopping may be prevented by adjusting the angle of a diffraction grating and/or the cavity length. For ultra-broadband, mode hop-free tuning, both diffraction grating angle tuning and cavity length tuning may be used in tandem.